The present invention pertains generally to commutators and more specifically to a device for minimizing crosstalk between channels of commutator multiplexers. Crosstalk has presented a serious limitation in the use of high impedance sources in conjunction with commutators which operate at a high rate, i.e., 25 khz to 100 khz and higher, due to inherent capacitance in the commutator switches. Since the commutator switches are connected in parallel to a single output, the capacitances of the switches add together to form a larger capacitance which is more capable of storing voltages and thereby increasing crosstalk between channels. Use of higher input impedance sources in conjunction with the commutator switches increases the RC time constant. Voltage stored on the inherent capacitance of the switches will not bleed off as quickly, thereby reflecting crosstalk voltage of a previous channel during the current switching cycle. In a commutator switching device, the actual crosstalk measurement is a function of the commutator input capacitance and the switching rate of the commutator. The commutator switches, as previously stated, have a finite input capacitance which is charged to the input signal level of the channel being sampled at a specific time. If the channel has a level of, for instance, 2.5 V, the inherent capacitance of the switches reflects a 2.5 V charge when the multiplexer switches to the next channel. If the next channel has an impedance of 500 kohms and a voltage level of -2.5 V, the stored +2.5 V level must be discharged through the 500 kohm resistor and recharged to the new -2.5 V level. In a high rate commutator, the inherent capacitance does not have time to fully discharge and subsequently recharge to the new source voltage level. The resulting error in voltage of the channel being sampled, due to previously sampled data, is defined as the interchannel crosstalk.
Existing commutator designs have minimized crosstalk by limiting the input capacitance. A standard approach has been to build at least two tiers of switches, the first tier containing a series of subgroups of the multiplexed channels. For example, in a 64-channel multiplexer, eight separate subgroups have been used, each containing eight channel multiplexers. A series of selection switches are used to connect each of the subgroups to the single output successively. In this manner, any particular signal source reflects the inherent capacitance of the switches within its own subgroup in addition to the capacitance of the series of additional successive switches. For example, in the 64-channel multiplexer any particular channel looks at the inherent capacitance of the eight switches within its group plus the eight successive selection switches, amounting to the inherent capacitance of 16 switches. This system therefore reduces the inherent capacitance of an unaltered system by one-fourth. However, this system adds to the complexity of the switching network in addition to adding additional costs relating to the additional switches required for operation of the system and required switching control means. In addition, the reliability and usefulness of such a system is reduced because of its complexity and implementation. As such, existing commutators on the market have been limited to a maximum input impedance of approximately 10 to 50 kohms because of the interchannel crosstalk.
For these reasons, expensive active signal conditioning networks have been incorporated between the high output impedance source and the commutative switches to reduce the output impedance of the source. The commutative multiplexing system requires that the output impedance of each of the sources be approximately equal. The standard method of matching impedances at the output is to use a series resistor and an RC tank network to ground. However, the Thevenin equivalent of the resistances normally reflects a high output impedance which is incompatible with the commutative switching network. Active signal conditioning networks are therefore required to present a low output impedance. Each of these active signal conditioning networks require expensive operational amplifiers and voltage supplies which greatly increase the overall cost of the system. Their complexity, in addition, introduces a reduced reliability factor in the operation of the system, as well as adding to the overall bulk of the system.